Sulfate-containing aqueous solutions incur as waste products in the various chemical-industrial processes. Disposal of such solutions is necessary. At present, they usually are evaporated for this purpose, the salts being used in other applications, if possible, in the solid state after comprehensive purification and drying procedures.
With an environment-conscious process control, the chemical circulation ought to be closed by recycling the sulfates into the primary process. For instance, in the xanthogenate process, this would mean that the spent spin bath liquor must be re-processed to sulfuric acid and lye so as to be able to recycle the sulfuric acid to the preparation of fresh spin bath liquor and the lye to the production of xanthogenate.
Attempts have been made for years to develop an electrolytic process by which such processing is feasible in a cost-saving manner. In SU-A - 701 961, for instance, a process is described according to which the splitting of sulfate is effected by aid of a three-compartment electrodialyzer. That is an electrolytic apparatus subdivided into three compartments by an anion and a cation exchange membrane, in which Na.sub.2 SO.sub.4 is electrolytically split into the splitting products H.sub.2 SO.sub.4 and NAOH, each in the form of an aqueous solution. The Na.sub.2 SO.sub.4 solution is fed into the intermediate compartment and the more or less concentrated output solutions are drawn off the respective electrode space.
An electrodialyzer of similar structure is described in DE-A- 3 529 649, in which, however, no pure Na.sub.2 SO.sub.4 solutions in water, but Na.sub.2 SO.sub.4 and H.sub.2 SO.sub.4 -containing solutions, as they incur, e.g., in the viscose fiber industry, are electrolyzed. The Na.sub.2 SO.sub.4 -containing solutions either are conducted in total into the intermediate compartment and from there into the anode compartment or are divided into two partial streams, which are fed into the two chambers simultaneously and then are united again.
However, the impurities contained in the spin bath liquor to be processed make both the membranes and the anodes last extremely short. Thus, for instance, the cathode-side membrane gets irreversibly damaged already within a few weeks due to a content of calcium ions that cannot be eliminated from the acidic solution by means of cation exchangers, which results not only in an increase in the voltage and a drop of the current efficiency, but also in a strongly growing permeability to sulfate ions.
Moreover, even the expensive electrocatalytic coating of the anode is attacked by a high concentration of organic impurities that cannot be removed to a sufficient extent, neither by commercially available activated carbons nor by adsorptive resins, which is reflected in an initially slow, then rapidly increasing rise in the voltage.
To electrolyze solutions containing Na.sub.2 SO.sub.4, H.sub.2 SO.sub.4 and alkaline earth ions, a process is known from EP-A 0 124 087, which is operated with a three-compartment cell consisting of two cation exchange membranes. In this electrolytical apparatus, the anode space is separated from the intermediate compartment by a cation exchange membrane and not by an anion exchange membrane as is the case with the above-described electrodialyzers. Electrodialysis is performed in a manner that the solution to be processed solely is fed into the anode chamber, while Na.sub.2 SO.sub.4 solution free of alkaline earth ions is introduced into the intermediate chamber. However, because of the alkaline earth ions (mostly Ca.sup.2+ and Mg.sup.2+) containuously penetrating through the cation membrane from the anode side, the constant purification of this intermediate chamber solution, which serves as a buffer, is required. Such a purification step has proved rather expensive.
A special form of the electrodialysis of Na.sub.2 SO.sub.4 solutions is described in EP-B - 0 096 239, according to which the water splitting basically required for salt splitting is effected not on electrodes, but on bipolar ion exchange membranes. These are laminates each comprised of a cation and an anion blocking layer, which allow for the progressive dissociation of water into H.sub.3 O.sup.+ and OH.sup.- ions in the electric field. Since this process does not involve the potential-consuming electrode procedures inherent in the simultaneous formation of H.sub.2 and O.sub.2, the bipolar membrane process is operated at a considerably lower current consumption than electrolysis.
In accordance with the mode of functioning of the bipolar membrane, the electrochemical cells are delimited by bipolar membranes instead of electrodes, the intermediate chamber, as in electrolysis, being subdivided into an acid, a base and, if desired, one or two salt compartments by at least one, if suitable, even two or three, ion exchange membranes.
A further development of the electrodialytical process described in the above-mentioned SU-A - 701 961 can be taken from SU-A - 916 601; yet, in order to achieve a better rate of decomposition of Na.sub.2 SO.sub.4 into its decomposition products, five three-compartment electrodialyzers plus one two-compartment electrolyzer are connected in a manner so as to be consecutively passed by the electrolytic solutions, the Na.sub.2 SO.sub.4 solution being conducted through the intermediate compartment of the electrodialyzers. Anion and cation exchange membranes are used as the membranes.
In addition to the desired increase in the decomposition rate from 40% with a single cell ) to 96-97%, also an increase in the current yield from 40-46% to 70% is automatically achieved.
All the electrodialytic processes known have the disadvantage that their current yield is relatively low and that damage to the membranes is frequent in the electrodialysis of alkali sulfate solutions containing alkaline earth ions, such as spent spin bath liquors from the xanthogenate process. Hence, the service lives of individual apparatus parts are undesiredly short.